The present invention relates to expanded resin beads. Particularly, the present invention relates to expanded resin beads having excellent fusion-bonding properties, therefore capable of lowering the molding temperature in production of in-molded articles and also capable of providing the molded products having excellent mechanical and thermal properties.
Expanded resin beads are used widely as molding material for a variety of molded products such as insulators, cushionings, cores, etc., because these resin beads are capable of assuming any desired shape and also low in heat conductivity due to their closed-cell structure. As thermoplastic resin constituting these expanded resin beads, polyethylene, polypropylene, polystyrene and the like are usually used.
In case where a high-melting point resin such as polypropylene-based resin is used for the expanded resin beads, however, high-pressure steam of over 2 kg/cm.sup.2 G(Gauge) is required for providing a pressure needed for conducting fusion-bonding of the expanded resin beads to each other during in-mold operations because the melting point of the said type of resin is usually not less than 135.degree. C. The above molding condition causes the disadvantages such as increased molding cost and prolonged molding cycle. Also, in the case of the expanded resin beads composed of a high-melting point resin such as mentioned above, a molding machine of high clamping pressure equipped with a high-pressure steam control system is required because the said type of resin beads are incapable of molding with an in-mold expansion molding machine for expanded polystyrene which is now popularly used in the art.
On the other hand, polyethylene type resins, have an advantages in which the steam pressure required for mutual fusion-bonding of the expanded resin beads can be low (below 2 kg/cm.sup.2 G) because the melting point of the polyethylene type of resins is low (below 125.degree. C.), and in which a molding machine for expanded polystyrene can be used with substantially no need of changing its specification. However, the expansion molded products of the polyethylene type resins are low in heat resistance because of low melting point of such resins, and especially the highly expanded molded articles are small in energy absorbing capacity. Therefore, practical application of the expansion molded products made from the polyethylene type resin has been limited to low-expansion uses, and there is a disadvantage of these products in comparison with the expansion molded articles made of other thermoplastic resins.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that the expansion molded articles are phenomenally improved in mechanical and thermal properties by using expanded resin beads which comprise a core comprising a specific resin, the said core being in an expanded state, and a coat comprising a specific polymer, the said coat being in a substantially non-expanded state. The present invention has been attained on the basis of the above finding.